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From Oleksandr Shulgin <oleksandr.shul...@zalando.de>
Subject Re: very slow repair
Date Thu, 13 Jun 2019 13:01:18 GMT
On Thu, Jun 13, 2019 at 2:09 PM Léo FERLIN SUTTON
<lferlin@mailjet.com.invalid> wrote:

> Last, but not least: are you using the default number of vnodes, 256?  The
>> overhead of large number of vnodes (times the number of nodes), can be
>> quite significant.  We've seen major improvements in repair runtime after
>> switching from 256 to 16 vnodes on Cassandra version 3.0.
>
>
> Is there a recommended procedure to switch the amount of vnodes ?
>

Yes.  One should deploy a new virtual DC with desired configuration and
rebuild from the original one, then decommission the old virtual DC.

With the smaller number of vnodes you should use
allocate_tokens_for_keyspace configuration parameter to ensure uniform load
distribution.  The caveat is that the nodes allocate tokens before they
bootstrap, so the very first nodes will not have keyspace information
available.  This can be worked around, though it is not trivial.  See this
thread for our past experience:
https://lists.apache.org/thread.html/396f2d20397c36b9cff88a0c2c5523154d420ece24a4dafc9fde3d1f@%3Cuser.cassandra.apache.org%3E

--
Alex

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